The “shell process” for making foundry shapes is well known. The shell process uses a phenolic novolak resin to coat sand and hexamethylene tetramine as the curing catalyst. The coated sand is used for making foundry shapes by filling a heated pattern or corebox with the coated sand, allowing the coated sand to cure for a period of time. Then the tooling (e.g. corebox, pattern, mold, etc.) is inverted to allow the excess uncured sand to fall away, leaving a shell of cured coated sand. The process is particularly useful for producing hollow cores.
The tooling is either hot when it is filled with the coated sand or it is heated after the coated sand is added, such that the temperature of the corebox or pattern typically ranges from 200° C. to 300° C. The heat catalyzes a chemical reaction between the hexamethylene tetramine and the novolak resin and the coated sand begins to cure. The cured shell is removed from the tool and used to cast metal parts.
The sand is typically coated by two different methods. One method involves coating the sand particles with the phenolic novolak resin, which is dispersed in an organic solvent, e.g. methanol. The solvent evaporates after the phenolic novolak resin and sand are mixed. Powdered or an aqueous solution of hexamethylene tetramine is added to coated sand before the solvent has completely evaporated.
The other method involves using a solid phenolic novolak resin to coat the sand. The solid phenolic novolak resins are typically added to hot sand and mixed. The heat melts the resin, which allows the resin to coat the surface of the sand grains with the phenolic novolak resin. Thereafter, an aqueous solution of hexamethylene tetramine is mixed with the coated sand. As the mixture cools and the water evaporates, the phenolic novolak resin solidifies on the sand particles. Continued agitation of the sand particles breaks up any lumps that may have formed and forms a free-flowing mixture of coated sand grains.
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